Reactor for reducing the contents of nitrogen oxides and sulphur oxides in combustion gases

ABSTRACT

A reactor for reducing the contents of nitrogen oxides and sulphur oxides in combustion gases. The reactor is in the form of a post-combustion chamber to be connected after a combustion chamber. The reactor has a casing including a generally cylindrical main part transforming at its top into a dome-shaped outlet part with an outlet opening. Around the casing of the reactor, there is provided a heat-insulated wall whose inner side has substantially the same shape as the casing and which is eccentrically disposed in relation to the casing. The casing of the reactor accommodates a partition which is conical and has its apex directed towards the outlet opening. A gap is formed between the shell and the partition. At the inlet end of the shell, an inlet funnel is provided at a certain distance from the inlet end as so to form a gap. A heat exchanger is provided for preheating secondary air supplied through a secondary air intake into the gap between the casing and the wall at a certain distance from the opening. At the bottom, the gap is connected to a collecting box and an outlet pipe.

BACKGROUND OF THE INVENTION

The present invention relates to a reactor for reducing the contents ofnitrogen oxides and sulphur oxides in combustion gases, which reactorcomprises a post-combustion chamber to be connected together with orafter a combustion chamber.

A major problem in the combustion of liquid and solid fuels is thecontent of sulphur oxides and nitrogen oxides present in the flue gases.Thus, many attempts have been made to reduce this oxide content, both byflue gas cleaning and by catalytic treatment of the exhaust gases.

The invention is based on the insight that it is possible to reduce thecontent of nitrogen oxides and sulphur oxides to a considerable extentif it is ensured that suitable oxidation and temperature conditionsprevail in the passageway between the combustion chamber and thechimney.

Swedish Patent 7804761-0 (SE-B-413,158) discloses an apparatus for thecombustion of a mixture of gaseous or particulate, combustible materialand combustion air. This apparatus is used for combusting variousgaseous or particulate materials containing carbon or carbon compounds,in such a complete manner that the combustion gases emitted arepractically free from soot, carbon monoxide and hydrocarbon residues. Itis, however, not stated in the patent specification that the apparatuscan be used for reducing the contents of nitrogen oxides and sulphuroxides in combustion gases.

U.S. Pat. No. 4,481,889 discloses a method for afterburning flue gasesby conducting the impure gases through a burner in an afterburner inwhich the exhaust gases, by being positively mixed with a combustiongas, are subjected to complete combustion. In this process, combustiblegases are thus supplied to bring about afterburning of the flue gases.

DE-A-3,014,590 discloses a pre-combustion chamber for an oil- orgas-fired, fan-supported burner. This pre-combustion chamber serves toshape the generated flame and to retard it before entering thecombustion chamber. This apparatus thus serves as an intermediarybetween the burner and the combustion chamber, whereas not as a reactorfor reducing the contents of nitrogen oxides and sulphur oxides incombustion gases.

SUMMARY OF THE INVENTION

A reactor is provided for reducing the contents of nitrogen oxides andsulphur oxides in combustion gases. The reactor is in the form of apost-combustion chamber to be connected after a combustion chamber. Thereactor has a casing including a generally cylindrical main parttransforming at its top into a dome-shaped outlet part with an outletopening. Around the casing of the reactor, there is provided aheat-insulated wall whose inner side has substantially the same shape asthe casing and which is eccentrically disposed in relation to thecasing. The casing of the reactor accommodates a partition which isconical and has its apex directed towards the outlet opening. A gap isformed between the shell and the partition. At the inlet end of theshell, an inlet funnel is provided at a certain distance from the inletend so as to form a gap. A heat exchanger is provided for preheatingsecondary air supplied through a secondary air intake into the gapbetween the casing and the wall at a certain distance from the opening.At the bottom, the gap is connected to a collecting box and an outletpipe.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail hereinbelow withreference to the accompanying drawings illustrating two embodiments ofthe device according to the invention.

In the drawings

FIG. 1 is a vertical section schematically showing an embodiment of thereactor according to the invention;

FIG. 2 is a section taken along the line II--II in FIG. 1.

FIG. 3 is a vertical section schematically showing an incineration plantusing another embodiment of a reactor according to the invention; and

FIG. 4 shows yet another embodiment of a reactor according to theinvention.

DETAILED DESCRIPTION

The arrangement shown in FIG. 1 comprises a reactor for reducing thecontents of nitrogen oxides and sulphur oxides in combustion gases. Thereactor has a casing or wall 10 with a substantially vertical, generallycylindrical shell 11 and a dome-shaped outlet end 12 associatedtherewith. The dome-shaped outlet end has a central outlet opening 13.The opposite end of the shell 11 forms an inlet end 14. Inside thecasing 10, there is provided a conical partition 15 which has its apexdirected towards the outlet end 13 and which is mounted on supportmembers 16 in a manner to define an annular gap 17 between the partition15 and the casing 10. Instead of an annular gap, the connection betweenthe upper and the lower part of the casing 10 may be in the form of atleast two edge recesses distributed around the periphery of thepartition, suitably as disclosed in SE-B-413,158 which is included byreference. At the bottom of the reactor, there is provided an inletfunnel 18 which leads the exhaust gases from a combustion chamber (notshown) into the reactor, so that the exhaust gases will be introduced ata suitably high velocity and directed towards the conical inner side ofthe partition 15. Around the casing 10, there is provided a furthercasing or wall 20 which has substantially the same shape as the casing10 but larger dimensions so as to define a gap 21 between the casings 10and 20. The casing 10 is eccentrically disposed in the casing 20. Thecasing 20 may be made of a heat-insulating material, but may also besurrounded by such a material. In the illustrated embodiment, anexternal heat-insulation 22 is used for the casing 20. The gap 21between the two casings is connected at the bottom to an annularcollecting box 23 connected to an outlet pipe 24, e.g. a chimney.

In the gap 21 between the two casings, there may be provided a heatexchanger (not shown in more detail) for preheating secondary air. Inthe embodiment according to FIG. 1, secondary air is however suppliedthrough an annular space 40 formed between the casing 20 and theexternal heat-insulation 22. The preheated secondary air is fed througha secondary air intake 25 into the space between the two casings at somedistance from the outlet opening 13.

Between the lower edge of the inlet end 14 of the inner casing 10 andthe inlet funnel 18, there is defined an annular gap 19 for theseparation of ash particles which have been separated in thepost-combustion chamber 10 or formed during the combustion therein.

When using the arrangement according to FIGS. 1 and 2, it isadvantageous to have the exhaust gases from the combustion chamberarrive in the inlet funnel 18 at a maximum velocity of 2 m/s. By theconical shape of the inlet funnel, the gas velocity is increased and thegases are directed towards the inner side of the conical partition 15.As a result of the intense turbulence in the space below the conicalpartition, residual carbon monoxide will oxidise into carbon dioxide,and this oxidation will proceed in the space above the partition. Fromthe outlet opening 13, the flue gases enter into the gap between thecasings 10 and 20 where afterburning and treatment of sulphur oxides andnitrogen oxides is performed under the action of the preheated secondaryair which is supplied through the secondary air intake 25 and preferablyheated to a temperature of about 700° C. By the eccentric arrangement,intense mixing is achieved as well as compression alternating withexpansion of the flue gases which are moving helically downwards to thecollecting box 23 before passing out to the outlet pipe or chimney 24 ata temperature of about 900° C.

The principle of the inventive device is based on experiments with idealturbulence for final oxidation of all hydrocarbon materials with acontrolled low partial pressure in the gas phase to achieve a sufficientcontact time with hot catalytic contact surfaces. The hot contactsurfaces initially consist of the material in the partition 15. Behindthis concave partition, there is thus a slower turbulence in a reducingatmosphere in order to obtain the necessary production of carbonmonoxide for the process, e.g. for reducing the sulphur content in thecombustion gases. In stoichiometric combustion and according to thefollowing formulae, sulphur deposits by more than 90% as sulphur dropletwhich have been sublimated during the cooling. Since the post-combustionchamber is vertically mounted, the sublimated sulphur, together withother particles, will automatically pass to the ash bed through the gapbetween the inlet funnel 18 and the inlet end 14.

When the post-combustion chamber is used in large-scale plants, theformula 2CO+SO₂ ⃡S+2CO₂ applies.

For plants with over-stoichiometric combustion, formulae CO+O₂ ⃡CO+CO₂and SO₂ +CO+H₂ O⃡H₂ S+CO and SO₂ +H₂ S⃡S+H₂ O apply.

If the gases entering the post-combustion chamber have a temperature of900° C. and a flow velocity of at most 2 m/s, it is possible to obtainsubstantially soot- and particle-free exhaust gases when a catalysingsurface exists on the conical partition 15 and on other contact surfacesaffecting the combustion gases.

The different formulae relating to the combustion chamber appear fromthe following.

The device according to the invention as illustrated in FIG. 3 hassubstantially the same design as that in FIG. 1. The device in FIG. 3 isshown together with an incineration plant of the type disclosed inSwedish Patent 7804761-0 (SE-B-413,158). For a more detailed descriptionof this arrangement, reference is thus made to said patent specificationwhich is included by reference. The device in FIG. 3 is generallydesignated 30. After this incineration device, there is a furthercombustion chamber 31 in which noxious waste or solid fuels, forinstance, can be combusted. From this combustion chamber or furnace 31,the combustion gases flow through a gap 32 up to the inlet funnel 18 andinto the post-combustion chamber according to the invention. The gap 32is formed between the incineration device 30 and a heat-insulatedfurnace wall 33. At the lower end of the space defined by the furnacewall 33, there is an ash outlet 34. Since the post-combustion chamber orreactor in FIG. 3 is essentially designed as in FIG. 1, equivalent partshave been given the same reference numerals. In the embodiment shown inFIG. 3, the partition 15 extends as far as the inner side of thecylindrical shell surface 11, and edge openings are provided whichextend obliquely through the partition 15 adjacent the shell surface,such that the passage between the space below the partition and thespace above it imparts a helical motion to the flue gases when enteringthe upper chamber above the partition 15.

FIG. 4 shows a further embodiment of a reactor according to the presentinvention. Corresponding parts have been given the same referencenumerals. The essential difference between the embodiments of FIG. 1 andFIG. 4 is the way of supplying secondary air through a secondary airintake 45. In this embodiment, the secondary air intake 45 consists of agap between two conical walls 40, 41. This gap is fed with secondary airwhich may have been preheated in any suitable manner. The air is eitherblown through the gap 45 or sucked therethrough as a result of theejector effect produced by the exhaust gases entering the reactorthrough the inlet funnel 18.

In the embodiment of FIG. 4, the conicial partition 15 has been designedin the manner shown in the above-mentioned SE-B-413,158, which meansthat there are provided at least two through passages 17 formed of edgeopenings distributed around the circumference of the partition andextending obliquely therethrough so as to impart a turbulent effect tothe flue gases when passing between the inlet chamber and the outletchamber.

The reactor according to the invention may advantageously be used alsoin incineration plants operating with a fluidised fuel bed.

    ______________________________________                                        I   INCOMPLETE COMBUSTION                                                     (CH.sub.2)n oil, gas                                                                   +     n O.sub.2 oxygen                                                                        ##STR1##                                                                            n CO carbon monoxide                                                                 +   n H.sub.2 O water vapour            II  CONVERSION                                                                CO carbon monoxide                                                                     +     H.sub.2 O water vapour                                                                  ##STR2##                                                                            CO.sub.2 carbon dioxide                                                              +   H.sub.2 hydrogen gas                METHANISING                                                                   CO carbon monoxide                                                                     +     3H.sub.2 hydrogen gas                                                                   ##STR3##                                                                            CH.sub.4 methane                                                                     +   H.sub.2 O water vapour              III COMPLETE COMBUSTION                                                       (CH.sub.2)n  oil, gas                                                                  +                                                                                    ##STR4##                                                                               ##STR5##                                                                            n CO.sub.2 carbon dioxide                                                            +   n H.sub.2 O water vapour            CO carbon monoxide                                                                     +                                                                                    ##STR6##                                                                               ##STR7##                                                                            CO.sub.2 carbon dioxide                        H.sub.2 hydrogen gas                                                                   +                                                                                    ##STR8##                                                                               ##STR9##                                                                            H.sub.2 O water vapour                         CH.sub.4 methane                                                                       +     3O.sub.2 oxygen gas                                                                     ##STR10##                                                                           CO.sub.2 carbon dioxide                                                              +   2H.sub.2 O water vapour             S sulphur                                                                              +     O.sub.2 oxygen                                                                          ##STR11##                                                                           SO.sub.2 sulphur dioxide                       2 CO     +     SO.sub.2                                                                                ##STR12##                                                                           S      +   2 CO.sub.2                          ______________________________________                                        Alternative reaction with excess of O.sub.2 and H.sub.2 O:                     ##STR13##                                                                     ##STR14##                                                                     ##STR15##                                                                

I claim:
 1. A reactor for reducing nitrogen oxides and sulphur oxidespresent in a combustion gas outlet stream from a combustion chamber,comprising:a casing comprising a vertically oriented, generallycylindrical shell having a longitudinal axis, a downwardly opening inletend arranged to receive said combustion chamber combustion gas outletstream and a generally dome-shaped outlet end axially opposite to saidinlet end, said outlet end having radially centrally located thereon anoutlet opening; a partition wall disposed within said casing at alocation intermediate said inlet end and said outlet opening anddividing said casing into an inlet chamber adjacent said inlet end, andan outlet chamber adjacent said outlet end; said partition wall beinggenerally transversally extending within said casing, so as to have acenter disposed generally radially centrally of said generallycylindrical shell, and an outer perimeter disposed near an internalperipheral sidewall surface of said generally cylindrical shell; meansdefining at least one opening from said inlet chamber into said outletchamber adjacent said sidewall surface of said generally cylindricalshell and remotely of said center of said partition wall; meansproviding a heat-insulating wall externally spaced from and enclosingsaid casing, thereby defining a space annularly between said wall meansand said cylindrical shell and terminally between said wall means andsaid outlet end; means defining a secondary air supply conduit meanshaving an outlet into one of:said space, at a location spaced from saidoutlet opening; and said inlet chamber; an outlet pipe; meanscommunicating said space, adjacent said inlet end of said casing, withsaid outlet pipe; a frusto-conically tapering inlet funnel projectingthrough said inlet end, into said inlet chamber of said casing, saidinlet funnel having a larger diameter inlet end disposed axially beforesaid inlet end of said shell and an outlet end disposed within saidinlet chamber, under said partition wall, whereby said combustion gasoutlet stream, in flowing through said inlet funnel into said inletchamber will be progressively reduced in transverse cross-sectionalarea; said inlet funnel, where axially passing said inlet end of saidshell, being located generally radially centrally of said casing andbeing smaller in diameter than said inlet end of said shell, therebydefining an open annular gap for egress of ash particles from withinsaid casing.
 2. The reactor of claim 1, wherein:said at least oneopening from said inlet chamber into said outlet chamber is constitutedby a plurality of openings formed obliquely through a perimetricallyouter portion of said partition wall in such a sense as to impart aswirling motion about said longitudinal axis to gas flowing from saidinlet chamber into said outlet chamber through said openings.
 3. Thereactor of claim 1, wherein:at least one of said shell, said outlet endof said casing and said partition wall contains a material having anability to catalyze oxygen of carbon and carbon compounds.
 4. Thereactor of claim 1, further including:means effective upstream of saidsecondary air outlet, for preheating secondary air flowing through saidsecondary air supply conduit means.
 5. The reactor of claim 1,wherein:said secondary air supply conduit means has an inlet openingdisposed adjacent said inlet end of said inlet funnel, said outlet ofsaid secondary air supply conduit being disposed adjacent said outletend of said inlet funnel; said secondary air supply conduit, betweensaid inlet end and outlet of said secondary air supply conduit extendingexternally upon said inlet funnel.